130 TEXTBOOK OF BOTANY 



of the protoplasm that man has not yet been able to duplicate in test 

 tubes. 



A reaction that may proceed in either of two directions is said to be 

 reversible, and is often represented by two arrows. A longer arrow is 

 usually used to indicate the direction in which the process is proceeding 

 more rapidly. 



(A) Glucose ^^^ H2O + Starch 



(B) Glucose T— =^ H2O + Starch 



Statement (A) above indicates that the condensation of glucose to 

 starch is proceeding more rapidly than the digestion of starch to glucose; 

 statement (B) indicates that digestion is the more rapid process. 

 Whether digestion will exceed condensation, or vice versa, at any par- 

 ticular time depends upon certain conditions in the plant cell, such as 

 acidity, water content, and the concentration of glucose. External factors, 

 such as temperature and light, affect these internal conditions and thus 

 indirectly affect the digestion and condensation processes. For instance, 

 when Irish potatoes are stored in bins at 10^-30° C. the sugar in them is 

 mostly condensed to starch; but when they are stored at temperatures 

 either above 30° C. or near freezing much of the starch is digested to 

 sugar. Potato chips prepared from potatoes in which sugar has accumu- 

 lated are dark in color. 



When the chloroplasts are exposed to light more sugar is made in 

 them and some of it condenses to starch. On the other hand, when the 

 guard cells of leaves are exposed to light the conditions initiated in them 

 by the light result in a change of starch to sugar. 



These changes of sugar to starch and of starch to sugar mav occur 

 repeatedly in the same cell, and at any time during the day or night. 

 Many efforts have been made to detect the amount of sugar that must 

 accumulate in a cell before some of it begins to condense to starch. 

 This amount of course is variable, depending both upon the kind of 

 plant and upon the conditions to which it is exposed. The formation of 

 starch is dependent upon the hereditary make-up of the plant. Closely 

 related plants may differ greatly in this respect, and in some plants 

 ( onion, hyacinth, and many others ) starch is formed only under excep- 

 tional environmental conditions, or not at all. 



The number of species of plants in which starch never forms under 

 any environmental condition is unknown. It is very simple to remove 



